MRI systems for performing whole body imaging usually employ large magnets which effectively surround the patient. Such magnets are usually large superconductor magnets which are expensive and difficult to maintain. MRI systems for performing local imaging of specific body parts or organs are known in the art.
U.S. patent application Ser. No. 08/898,773, now U.S. Pat. No. 5,900,793 to Katznelson et al., filed Jul. 23, 1997 and entitled "PERMANENT MAGNET ASSEMBLIES FOR USE IN MEDICAL APPLICATIONS", now U.S. Pat. No. 5,900,793 and incorporated herein by reference discloses, inter alia, compact permanent magnet assemblies for use in medical applications including MRI and/or MRT.
A typical application using an intra-operative MRI system is brain surgery. Reference is now made to FIG. 1 which is a schematic perspective view of a small organ dedicated MRI probe useful in brain surgery. The MRI probe 1 includes two annular permanent magnet assemblies 2 and 4 connected by a frame 3. The frame 3 and the magnet assemblies 2 and 4 are shaped for imaging the brain of a patient 6. During MRI assisted brain surgery or MRT, the head of the patient 6 is positioned between the two magnet assemblies 2 and 4.
Reference is now made to FIG. 2 which is a schematic isometric view of the two permanent magnet assemblies 2 and 4 of FIG. 1. Each of the magnet assemblies 4 and 2 includes three preferably concentric annular permanent magnets 4a, 4b, 4c and 2a, 2b, 2c (not shown in drawing). The annular permanent magnets 4a, 4b and 4c are offset from each other along the axis 12, and the annular permanent magnets 2a, 2b and 2c (not shown) are also offset from each other along the axis 12 as disclosed in U.S. Pat. No. 5,900,793 to Katznelson et al., now U.S. Pat. 5,900,793.
The axis 12 is the axis of symmetry of both magnet assemblies 2 and 4, passing through their centers. The axis 12 coincides with the z-axis along which the main magnetic field generated by the magnet assemblies 2 and 4 is oriented.
In order to reduce eddy currents each one of the concentric annular permanent magnets 4a, 4b, 4c, 2a, 2b and 2c is formed from segments 24 each of which is permanently magnetized in a known manner and then attached to the neighboring segments using an electrically non-conducting glue (not shown) or non-conductive spacers (not shown). For example, the segments 24 can be made from a neodymium-iron-boron (Nd--Fe--B) alloy. However, the segments 24 can be made from any other alloy or ceramic material suitable for forming permanent magnets of sufficient magnetic field strength. Preferably, the material from which the segments 24 are made should have a relatively low electrical conductivity.
The magnet assemblies 2 and 4 joined together by frame 3 (not shown in FIG. 2) define a region 16 having therein a volume 18 of substantially uniform magnetic field, between the pair of magnet assemblies 2 and 4.
The MRI probe 1 further includes Gradient coils (not shown) for generating gradient fields, shim coils (not shown) for active shimming of the main magnetic field, RF coils (not shown), a temperature control system (not shown) and an RF shield (not shown).
Ordinarily, the gradient fields are generated by a set of coils, through which a current of an adequate magnitude flows. During the periods of building up and decay of the currents, the temporal change of the magnetic flux, originally generated by the currents, creates eddy currents in conductive materials situated in their vicinity such as soft iron parts or permanent magnet parts used in prior art MRI permanent magnets or the aluminum enclosures of the cooling systems used in super-conducting magnets of MRI systems. The eddy currents generated by the gradient coil magnetic flux changes, generate secondary magnetic fields which may interfere with the primary gradient fields and affect their precision in encoding the spatial information.
In prior art MRI devices, the gradient coils are located within the internal free volume situated in the main magnet, where the imaged body is also introduced. To attenuate the effect of the spurious eddy currents, prior art MRI devices may use shielded gradient coils or pre-emphasis circuits which modify gradient amplifier demand in order to compensate for eddy currents. In small organ dedicated MRI probes and in MRI probes adapted for intra-operative use such as the MRI probe 1 of FIG. 1, the dimensions of the region 16 (best seen in FIG. 2) for accommodating the organ to be imaged are limited by practical considerations. Generally, the design of such MRI systems involves a tradeoff between maximizing the intensity and homogeneity of the magnetic field in as large an imaging volume as possible and providing maximal accessibility of the surgeon to the organ undergoing surgery. For example, the MRI probe 1 (FIGS. 1 and 2) is designed to maximize the size of the volume 18 of homogenous magnetic field while keeping the size of the magnet assemblies 2 and 4 minimal while allowing enough space for positioning the shoulders of the patient 6. If one tries to increase the space available for the shoulders of the patient 6 by increasing the distance between the magnet assemblies 2 and 4 along the axis 12, the resulting decrease in the strength and homogeneity of the magnetic field will have to be compensated. The magnetic field can be compensated by increasing the thickness of the annular permanent magnets 4a, 4b, 4c of FIG. 2 and 2a, 2b and 2c (not shown in FIG. 2).
Increasing the thickness of the annular permanent magnets 4a, 4b, 4c, 2a, 2b and 2c (not shown) is practically limited since their magnetic field depends non-linearly on their thickness. Thus, increasing the thickness of an annular permanent magnet above a certain value, results in a negligible contribution to the magnetic field strength.
The magnetic field can also be compensated by increasing the size and diameter of the magnet assemblies 2 and 4. However, increasing the diameter of the magnet assemblies 2 and 4 may in turn shift the location of the volume 18 relative to the desired position of the head of the patient 6. The shifting may also prevent access to and imaging of the lower part of the brain, affecting the types of surgery that can be performed using the probe 1.
Thus, placing the gradient coils and/or shim and RF coils within the already restricted region 16 between the magnet assemblies 2 and 4, limits even further the space available for positioning the organ to be imaged and may hinder access to the organ undergoing surgery and the placing and manipulating of surgical instruments within that organ during surgery.
Furthermore, in MRI systems using permanent magnets, if the gradient coils are positioned in close proximity to the permanent magnets, the heat developed in the resistive gradient coils by the currents flowing within the coils may heat the permanent magnet. The heat generated by the gradient coils may thus cause local temperature increase in the permanent magnets. Such temperature changes are undesirable since the field generated by permanent magnets is highly susceptible to large variations induced by local temperature changes.
MRI systems based on permanent magnets such as the MRI probe 1 of FIG. 1 or the MRI probe of FIG. 2, do not include electrically conducting structures operating as magnetic flux return structures. This fact, in addition to the segmented structure of the annular permanent magnets 4a, 4b, 4c and 2a, 2b and 2c (not shown) and the intrinsic low conductivity of the Nd--Fe--B alloy from which they are made, substantially reduce the spurious eddy current problem.
Whole body MRI/MRT systems typically use a fixed installation RF cage for preventing magnetic, electromagnetic and electrical noise from the outside from penetrating into the imaging volume inside the probe and interfering with the weak NMR signals generated during imaging. In addition, the RF cage is also used to reduce the leakage of the RF radiation generated within the probe during imaging to prevent disturbances to other electrical devices used near the MRI probe.
Unfortunately, for practical reasons, large fixed installation RF cages or RF rooms cannot always be used small organ dedicated MRI or MRT probes of the type used for intra-operative imaging such as the MRI probe 1 of FIG. 1. For example, while the small organ dedicated MRI probe 1 may be operated within a large shielded RF room, this will necessitate the use of special expensive shielded surgical equipment that is designed to create minimal RFI disturbances so as not to interfere with the operation of the MRI probe 1.